transformers-0.5.5.0: Concrete functor and monad transformers

Copyright(c) Michael Weber <michael.weber@post.rwth-aachen.de> 2001
(c) Jeff Newbern 2003-2006
(c) Andriy Palamarchuk 2006
LicenseBSD-style (see the file LICENSE)
MaintainerR.Paterson@city.ac.uk
Stabilityexperimental
Portabilityportable
Safe HaskellSafe
LanguageHaskell98

Control.Monad.Trans.Error

Contents

Description

Deprecated: Use Control.Monad.Trans.Except instead

This monad transformer adds the ability to fail or throw exceptions to a monad.

A sequence of actions succeeds, producing a value, only if all the actions in the sequence are successful. If one fails with an error, the rest of the sequence is skipped and the composite action fails with that error.

If the value of the error is not required, the variant in Control.Monad.Trans.Maybe may be used instead.

Note: This module will be removed in a future release. Instead, use Control.Monad.Trans.Except, which does not restrict the exception type, and also includes a base exception monad.

Synopsis

The ErrorT monad transformer

class Error a where Source #

An exception to be thrown.

Minimal complete definition: noMsg or strMsg.

Methods

noMsg :: a Source #

Creates an exception without a message. The default implementation is strMsg "".

strMsg :: String -> a Source #

Creates an exception with a message. The default implementation of strMsg s is noMsg.

Instances

Error IOException Source # 
ErrorList a => Error [a] Source #

A string can be thrown as an error.

Methods

noMsg :: [a] Source #

strMsg :: String -> [a] Source #

class ErrorList a where Source #

Workaround so that we can have a Haskell 98 instance Error String.

Minimal complete definition

listMsg

Methods

listMsg :: String -> [a] Source #

Instances

newtype ErrorT e m a Source #

The error monad transformer. It can be used to add error handling to other monads.

The ErrorT Monad structure is parameterized over two things:

  • e - The error type.
  • m - The inner monad.

The return function yields a successful computation, while >>= sequences two subcomputations, failing on the first error.

Constructors

ErrorT 

Fields

Instances

MonadTrans (ErrorT e) Source # 

Methods

lift :: Monad m => m a -> ErrorT e m a Source #

(Monad m, Error e) => Monad (ErrorT e m) Source # 

Methods

(>>=) :: ErrorT e m a -> (a -> ErrorT e m b) -> ErrorT e m b #

(>>) :: ErrorT e m a -> ErrorT e m b -> ErrorT e m b #

return :: a -> ErrorT e m a #

fail :: String -> ErrorT e m a #

Functor m => Functor (ErrorT e m) Source # 

Methods

fmap :: (a -> b) -> ErrorT e m a -> ErrorT e m b #

(<$) :: a -> ErrorT e m b -> ErrorT e m a #

(MonadFix m, Error e) => MonadFix (ErrorT e m) Source # 

Methods

mfix :: (a -> ErrorT e m a) -> ErrorT e m a #

(Monad m, Error e) => MonadFail (ErrorT e m) Source # 

Methods

fail :: String -> ErrorT e m a #

(Functor m, Monad m) => Applicative (ErrorT e m) Source # 

Methods

pure :: a -> ErrorT e m a #

(<*>) :: ErrorT e m (a -> b) -> ErrorT e m a -> ErrorT e m b #

(*>) :: ErrorT e m a -> ErrorT e m b -> ErrorT e m b #

(<*) :: ErrorT e m a -> ErrorT e m b -> ErrorT e m a #

Foldable f => Foldable (ErrorT e f) Source # 

Methods

fold :: Monoid m => ErrorT e f m -> m #

foldMap :: Monoid m => (a -> m) -> ErrorT e f a -> m #

foldr :: (a -> b -> b) -> b -> ErrorT e f a -> b #

foldr' :: (a -> b -> b) -> b -> ErrorT e f a -> b #

foldl :: (b -> a -> b) -> b -> ErrorT e f a -> b #

foldl' :: (b -> a -> b) -> b -> ErrorT e f a -> b #

foldr1 :: (a -> a -> a) -> ErrorT e f a -> a #

foldl1 :: (a -> a -> a) -> ErrorT e f a -> a #

toList :: ErrorT e f a -> [a] #

null :: ErrorT e f a -> Bool #

length :: ErrorT e f a -> Int #

elem :: Eq a => a -> ErrorT e f a -> Bool #

maximum :: Ord a => ErrorT e f a -> a #

minimum :: Ord a => ErrorT e f a -> a #

sum :: Num a => ErrorT e f a -> a #

product :: Num a => ErrorT e f a -> a #

Traversable f => Traversable (ErrorT e f) Source # 

Methods

traverse :: Applicative f => (a -> f b) -> ErrorT e f a -> f (ErrorT e f b) #

sequenceA :: Applicative f => ErrorT e f (f a) -> f (ErrorT e f a) #

mapM :: Monad m => (a -> m b) -> ErrorT e f a -> m (ErrorT e f b) #

sequence :: Monad m => ErrorT e f (m a) -> m (ErrorT e f a) #

(Eq e, Eq1 m) => Eq1 (ErrorT e m) Source # 

Methods

liftEq :: (a -> b -> Bool) -> ErrorT e m a -> ErrorT e m b -> Bool #

(Ord e, Ord1 m) => Ord1 (ErrorT e m) Source # 

Methods

liftCompare :: (a -> b -> Ordering) -> ErrorT e m a -> ErrorT e m b -> Ordering #

(Read e, Read1 m) => Read1 (ErrorT e m) Source # 

Methods

liftReadsPrec :: (Int -> ReadS a) -> ReadS [a] -> Int -> ReadS (ErrorT e m a) #

liftReadList :: (Int -> ReadS a) -> ReadS [a] -> ReadS [ErrorT e m a] #

(Show e, Show1 m) => Show1 (ErrorT e m) Source # 

Methods

liftShowsPrec :: (Int -> a -> ShowS) -> ([a] -> ShowS) -> Int -> ErrorT e m a -> ShowS #

liftShowList :: (Int -> a -> ShowS) -> ([a] -> ShowS) -> [ErrorT e m a] -> ShowS #

(Error e, MonadIO m) => MonadIO (ErrorT e m) Source # 

Methods

liftIO :: IO a -> ErrorT e m a #

(Functor m, Monad m, Error e) => Alternative (ErrorT e m) Source # 

Methods

empty :: ErrorT e m a #

(<|>) :: ErrorT e m a -> ErrorT e m a -> ErrorT e m a #

some :: ErrorT e m a -> ErrorT e m [a] #

many :: ErrorT e m a -> ErrorT e m [a] #

(Monad m, Error e) => MonadPlus (ErrorT e m) Source # 

Methods

mzero :: ErrorT e m a #

mplus :: ErrorT e m a -> ErrorT e m a -> ErrorT e m a #

(Eq e, Eq1 m, Eq a) => Eq (ErrorT e m a) Source # 

Methods

(==) :: ErrorT e m a -> ErrorT e m a -> Bool #

(/=) :: ErrorT e m a -> ErrorT e m a -> Bool #

(Ord e, Ord1 m, Ord a) => Ord (ErrorT e m a) Source # 

Methods

compare :: ErrorT e m a -> ErrorT e m a -> Ordering #

(<) :: ErrorT e m a -> ErrorT e m a -> Bool #

(<=) :: ErrorT e m a -> ErrorT e m a -> Bool #

(>) :: ErrorT e m a -> ErrorT e m a -> Bool #

(>=) :: ErrorT e m a -> ErrorT e m a -> Bool #

max :: ErrorT e m a -> ErrorT e m a -> ErrorT e m a #

min :: ErrorT e m a -> ErrorT e m a -> ErrorT e m a #

(Read e, Read1 m, Read a) => Read (ErrorT e m a) Source # 

Methods

readsPrec :: Int -> ReadS (ErrorT e m a) #

readList :: ReadS [ErrorT e m a] #

readPrec :: ReadPrec (ErrorT e m a) #

readListPrec :: ReadPrec [ErrorT e m a] #

(Show e, Show1 m, Show a) => Show (ErrorT e m a) Source # 

Methods

showsPrec :: Int -> ErrorT e m a -> ShowS #

show :: ErrorT e m a -> String #

showList :: [ErrorT e m a] -> ShowS #

mapErrorT :: (m (Either e a) -> n (Either e' b)) -> ErrorT e m a -> ErrorT e' n b Source #

Map the unwrapped computation using the given function.

Error operations

throwError :: Monad m => e -> ErrorT e m a Source #

Signal an error value e.

catchError Source #

Arguments

:: Monad m 
=> ErrorT e m a

the inner computation

-> (e -> ErrorT e m a)

a handler for errors in the inner computation

-> ErrorT e m a 

Handle an error.

Lifting other operations

liftCallCC :: CallCC m (Either e a) (Either e b) -> CallCC (ErrorT e m) a b Source #

Lift a callCC operation to the new monad.

liftListen :: Monad m => Listen w m (Either e a) -> Listen w (ErrorT e m) a Source #

Lift a listen operation to the new monad.

liftPass :: Monad m => Pass w m (Either e a) -> Pass w (ErrorT e m) a Source #

Lift a pass operation to the new monad.

Examples

Wrapping an IO action that can throw an error e:

type ErrorWithIO e a = ErrorT e IO a
==> ErrorT (IO (Either e a))

An IO monad wrapped in StateT inside of ErrorT:

type ErrorAndStateWithIO e s a = ErrorT e (StateT s IO) a
==> ErrorT (StateT s IO (Either e a))
==> ErrorT (StateT (s -> IO (Either e a,s)))

Orphan instances

Error e => Alternative (Either e) Source # 

Methods

empty :: Either e a #

(<|>) :: Either e a -> Either e a -> Either e a #

some :: Either e a -> Either e [a] #

many :: Either e a -> Either e [a] #

Error e => MonadPlus (Either e) Source # 

Methods

mzero :: Either e a #

mplus :: Either e a -> Either e a -> Either e a #